1. Field of the Invention
The present invention relates to a flat panel display device, more specifically to a flat panel display device capable of reducing the ripple interference resulting from ground current.
2. Description of the Prior Art
A flat panel display device, especially a liquid crystal display (LCD), due to its low power dissipation, low radiation, and smaller volume than a traditional cathode ray tube (CRT) display, is widely used in notebook PCs, personal digital assistants (PDAs), cellular phones, clocks, etc, and even there is an increasing tendency to substitute the CRT monitor by a LCD for a desktop PC.
Please refer to FIG. 1, which shows a schematic diagram of a LCD device 10 according to prior art. The LCD device 10 includes a housing 12 and a plurality of circuits 14 such as an AC/DC adapter 14a for transforming and rectifying an AC voltage VAC into a DC voltage, a DC/AC inverter 14b for transforming a DC voltage into an AC voltage with a high frequency and a high voltage value for driving a discharge tube of a backlight module in the LCD device 10, a DC/DC converter 14c for transforming the DC voltage into a required operating voltage, a controller 14d for controlling the LCD panel of the LCD device 10, and other system circuits (not shown). Each circuit 14 includes a reference voltage end electrically connected with a reference voltage level. For example, a node B is the reference voltage end of the DC/AC adapter 14b. Because the housing 12 is a conductor electrically connected to a ground terminal Gnd,as shown in FIG. 1, the reference voltage end of each circuit 14 is connected to the housing 12. In other words, the reference voltage end of each circuit 14 is connected to the ground terminal Gnd so as to connect to a ground level in a normal operation. Moreover, a display driver 18 of a host 16, which is used to transmit data to the controller 14d, is connected to the controller 14d and thus the controller 14d will drive the LCD device 10 according to the data to be displayed so that the data to be displayed is displayed on the LCD panel.
Please refer to FIG. 2 and FIG. 3. FIG. 2 and FIG. 3 are schematic diagrams illustrating the status of a conventional LCD device operating under a burst dimming mode, where FIG. 2 shows a status of a conventional LCD device operating under a full-load current condition and FIG. 3 shows a status of a conventional LCD device operating under a half-load current condition. As far as the conventional display device is considered, a user can adjust a brightness of the display device based on his demand. However, the variation in brightness of the LCD device will become uniform under the burst mode dimming operation, in which pixels at different positions will generate identical variations in brightness during the brightness adjusting process. A principle of the burst mode dimming operation is summarized as follows. Assume that the controller 14d drives the LCD device 10 based on a current Ia, where the current Ia is defined as the brightness of the LCD device 10 (that is, 0-255 gray levels). In order to keep the gray level of the LCD device to be correspondent with the current Ia, the controller 14d will continuingly drive the LCD device 10 based on the current Ia, a shown in FIG. 2. However, when a user intends to adjust the brightness of the LCD device 10, for example when a user intends to reduce the original brightness of the LCD device 10 by half, the controller 14d still drives the LCD device 10 based on the current launder the burst dimming mode, but the duration for driving the controller 14d will be changed. As shown in FIG. 3, the controller 14d drives the LCD device 10 for only 1/400 second in every 1/200 second time period. That is, the current 1a can be considered as working at a frequency of 200 Hz with a duty cycle being tantamount to 50%. This means the controller 14d defines the brightness of the LCD device 10 as an equivalent to the current value of (½*Ia), so that the brightness setting of the LCD device 10 can be reduced. In this way, under bust dimming mode the current value Ia at a predetermined frequency (for example, 200 Hz) can be equivalently changed by adjusting its duty cycle so as to achieve brightness adjustment under burst dimming mode.
FIG. 4 is an equivalent circuit diagram of FIG. 1 showing the ground terminal Gnd being connected to a ground level. Ground can be regarded as an equivalent circuit constructed from a current source Ig connected in parallel with an inductor Lg. When the LCD device 10 is undergoing brightness adjustment under the burst dimming mode, because the housing 12 is connected to the ground terminal Gnd, a low-frequency ripple current Ig will be induced at the ground terminal Gnd to flow into the LCD device 10. The current Ig will influence a duty cycle of the current Ia, resulting in a corresponding fluctuation on the equivalent current Ia. The fluctuation of the brightness of a LCD device causes an output ripple wave on the display panel, leading to a bad display quality due to the ripple interference.